Western Interior Seaway

Western Interior Seaway during the mid-Cretaceous, about 100 million years before the present

The Western Interior Seaway[1] was a huge inland sea. It split North America into two halves for most of the mid- and late-CretaceousPeriod. It was up to 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long.

Sea level was high in the Cretaceous, and the middle part of North America sank down. A shallowly-subductingplate pulled on the bottom of the lithosphere, causing it to move down. The sea entered both north and south, and came and went during the course of the Cretaceous. Widespread carbonate deposition suggests that the Seaway was warm and tropical, with abundant calcareous algae.[2] The shrunken, regressive phase of the Western Interior Seaway is sometimes called the Pierre Seaway.[3]

The Western Interior Seaway was home to early birds also, including the flightless Hesperornis which had stout legs for swimming through water and small wing-like appendages used for marine steering rather than flight; and the tern-like Ichthyornis, an early avian with a toothy beak. Icthyornis shared the sky with large pterosaurs such as Nyctosaurus and Pteranodon. Pteranodon fossils are very common and it was likely a major component of the surface ecosystem, though it was only found in the southern reaches of the Seaway.[5]

On the bottom, the giant clamInoceramus left common fossilized shells in the Pierre Shale. This clam had a thick shell paved with "prisms" of calcite deposited at right angles to the surface, giving a pearly luster in life. Paleontologists suggest that its giant size was an adaptation for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen-depleted waters.